1. Field of the Invention
The present invention relates to a video head assembly for use in a transverse scanning video recording machine, and more particularly to a head assembly wherein the tape guide and the head wheel have been modified to produce a head assembly capable of a substantially increased lifetime.
2. Description Relative to the Prior Art
When selecting a video recording machine for use in applications wherein quality of recording is of prime importance and cost is a secondary consideration, one is likely to choose a video recording machine of the transverse scanning type. The mechanical heart of a transverse scanning video recording machine is the video head assembly. The head assembly comprises a rotary head wheel, a set of magnetic transducer heads mounted to the head wheel, and a vacuum guide having a surface over which a recording tape is advanced as it passes through the head assembly.
After investing in a transverse scanning recording machine, the user may generally enjoy up to 300 or 400 hours of satisfactory performance in a typical broadcasting environment. After about 350 hours of use, the head assembly must be disassembled, worn components replaced, and parts aligned to meet specifications; usually, the worn components are the transducer heads which have worn past the usable stage. In addition to having to replace the transducer heads every 350 hours, or thereabouts, the vacuum guide needs replacement or repair after about 1,000 hours of use. In other words, about every third time the transducer heads are replaced, the vacuum guide should also be replaced (or repaired). An improved video head assembly is obliged to anticipate failure due to all components therein, i.e., the transducer heads, the vacuum guide, and the head wheel.
In the present state of the art, the transducer heads are made of a magnetic metal, such as Alfesil or Sendust. The heads are mounted on a rotary head wheel generally constructed of chrome-plated metal. The vacuum guide commonly comprises a single piece of metal, such as aluminum or steel, machined to have a tape guide surface and necessary vacuum passageways to provide suction for holding the recording tape to the guide surface as it passes through the head assembly.
The magnetic metals used in the transducer heads are relatively soft and tend to wear rapidly, the average head life, as previously mentioned, being about 350 hours. Now it is well known that ferrite is a substantially harder material than the magnetic metals. In fact, the increased hardness of ferrite may allow a transducer head life time in excess of 1,500 hours. Also, the high frequency permeability of ferrite is much greater than that of metal, thereby enabling an improved frequency response to be obtained. As would be expected, attempts have been made to employ ferrite as the transducer head material. Unfortunately, in addition to being harder than the magnetic metals, ferrite is also more brittle. And, as a result, attempts to use ferrite as the head material have generally been unsuccessful, the brittle ferrite being unable to withstand the severe contact forces encountered in the head assembly.
In U.S. Pat. application Ser. Nos. 668,873 and 734,698, a transducer head is proposed which obtains the advantages of the ferrite head while providing protection for the brittle ferrite. Disclosed in the application is an improved transducer head comprising a ferrite head provided with ceramic support shoes, or inserts. It is this ceramic-ferrite combination which is able to withstand the harsh environment in a transverse scanning video recording machine. The ceramic evidently absorbs a major portion of the "pounding" and provides the support necessary to prevent the ferrite from shattering. Evidently, the substitution of ceramic-ferrite transducer heads for metal transducer heads in an otherwise unmodified head assembly should not present any particular problems, and the head assembly should now perform satisfactorily for about 1,500 hours (ignoring, for the moment, the fact that the vacuum guide generally fails after only 1,000 hours of operation). In actual practice, however, it would be expected that the "improved" head assembly would not perform adequately for the 1,500 hours expected of the transducer heads. The reason for this expected premature failure of the head assembly would not be apparent upon visual inspection of the ceramic-ferrite transducer heads, the head wheel, or the vacuum guide. Only with a miscroscope, sensitive micrometer, or similar device, would one perceive the source of the problem.
FIG. 1 shows a head wheel 10 having ceramic-ferrite transducer heads 12a, b, c, and d, as it would appear if taken from the head assembly after perhaps 1,000 hours of operation (about the time when performance may become unsatisfactory). The enlarged view from the area of the head wheel in the immediate vicinity of the ceramic-ferrite head 12a shows the problem. The ceramic-ferrite head 12a protects the portions 14a and b of the head wheel immediately adjacent thereto from wear, thereby causing a slight out-of-round wearing of the head wheel, the head wheel taking on a generally cruciform shape, having a small protuberance around each head. Although the out-of-roundness is not even visible to the unaided eye, it occurs on a critical surface. During operation, the head wheel rotates as a recording tape is advanced over a guide surface on the vacuum guide. The guide surface generally has a groove perpendicular to the direction of tape travel over which the tape passes and in which the head wheel slightly extends, and in which the transducer heads more significantly extend. In practice, the tape vibrates due to the periodic passes of the transducer heads mounted to the rotary head wheel. When the head wheel itself has worn out of round, this vibratory effect is not damped by the head wheel and, though not apparent to the naked eye, the recording tape literally "flaps" as it passes through the head assembly. As the tape "flaps", tape stretch is no longer constant and the information may be recorded with some amplitude modulation, degrading video performance. Therefore, to obtain better improvement in head assembly life time through the use of a ceramic-ferrite head, a head wheel is provided which is so designed as to resist wearing in a generally cruciform shape, or other out-of-round shape, during normal operation.
To realize the full advantage of a ceramic-ferrite head (assuming now, for the moment, that a head wheel may be provided which will not cause the head assembly to fail before the ceramic-ferrite heads themselves wear out), a vacuum guide must be provided which does not require repair or replacement before the ceramic-ferrite heads. A typical vacuum guide is shown in FIG. 2. As a tape is advanced over the guide surface 16 in the direction indicated by the arrow (the tape being held against the surface by vacuum suction via a pair of passageways 18 and 20), the surface wears. The result of such wear is to change the radius of curvature of the guide surface 16 to where it no longer matches the SMPTE standard. When this wear becomes severe, as indicated in an exaggerated manner by the dotted line 23, serious time base errors may be introduced and the vacuum guide should generally be replaced. As previously stated, this vacuum guide failure may occur before the ceramic-ferrite heads fail. Thus, an improved head assembly should take this factor into account by providing a vacuum guide more suitable for use with a head assembly employing ceramic-ferrite heads.